1. Field of the Invention
The present invention relates to a disk control unit, and more particularly to a disk control unit which controls concurrent access to a disk unit having a plurality of logical drives. The present invention applies advantageously to a magnetic disk unit, an optical disc unit, and a disk array unit.
2. Description of the Related Art
It is essential for a disk array unit to have a cache memory to increase throughput. Data in frequently-accessed areas, which are usually located only in a limited part of the disk array unit, is copied into the cache memory to respond to the host computer more quickly.
A RAID5 disk array performs specific processing, called write penalty, to prevent disk array performance from being decreased. Recently, as a disk array becomes larger, a larger cache memory is required to maintain throughput.
Another technology to increase throughput is implemented by a multi-director system composed of a plurality of directors which perform I/O processing in order to distribute the load among them. This multi-director system allows host directors to concurrently process I/O requests from a plurality of hosts or I/O requests issued concurrently from one host. In addition, the multi-director system allows a disk director to access physical disks independently within the disk array to asynchronously execute high-priority host-to-cache data transfer and background cache-to-disk data transfer, thus increasing throughput.
FIG. 7 shows a magnetic disk control unit disclosed in Japanese Patent Laid-Open Publication No. Hei 8-115257. This control unit has a disk cache memory that is divided into a plurality of blocks each with a corresponding write count management table. This table keeps track of block usage status to control access to disk cache blocks.
Although the multi-director system allows I/O processing to be executed concurrently, only one director can access the cache memory management area at a time. When using cache memory, each director searches, allocates, or exclusively controls the cache memory, or deletes data from it. In doing so, each director must process data in the cache memory management area exclusively. This means that, when a plurality of directors want to access the cache memory management area at the same time, only one director is allowed to do so and the remaining directors are put in the wait state. Therefore, an increase in the number of directors results in a conflict of requests for access to the cache memory management area, sometimes reducing the advantage of the multi-director system.
The present invention seeks to solve the problems associated with the prior art described above. It is an object of the present invention to provide a disk control unit and a disk control method which improve the usage of a cache memory in a disk unit to increase the speed of concurrent access to the cache memory.
It is another object of the present invention to provide a disk control unit and a disk control method which provide quick access to a disk array unit with a plurality of directors.
It is still another object of the present invention to provide a disk control unit and a disk control method which allow a plurality of hosts to access volumes, which are logical disks, at an equal speed.
The present invention provides a disk control unit comprising a plurality of directors each individually and independently controlling I/O processing between one of a plurality of hosts and a disk unit; a cache memory connected to the directors and having a plurality of cache areas provided according to a configuration of the disk unit; and a plurality of cache management areas each provided for each cache area for checking if the cache area is used by one of the directors. In addition, an exclusive control unit is provided to cause each of the directors to reference the cache management area to exclusively control the cache area.
Each director performs I/O processing between the host and the disk unit via the cache memory. At this time, each director performs its own I/O processing independently of I/O processing performed by other directors. That is, the directors perform concurrent I/O processing. The director references the cache management area to exclusively control the cache area. Therefore, even when one director uses one cache area, other directors may use other cache areas concurrently. This disk control unit makes it possible for a plurality of directors to make faster concurrent processing.